This invention relates to an optical drop-and-insert apparatus which can be applied to an optical fiber network, and more particularly to an optical drop-and-insert apparatus which has an optical dropping function for selectively extracting desired optical data from an optical data pulse train on an optical highway multiplexed on the time base and sending the thus extracted optical data to an optical terminal and an optical inserting function for placing optical data from an optical terminal onto an empty time slot of the optical highway.
As development of an optical communication network progresses, various investigations are made for a system of transmission and reception of optical signals such as realization of an optical switching system, building of a system of the optical subscriber type or the like. Above all, realization, for example, of an optical subscriber transmission network with which an optical fiber transmission line is introduced into a subscriber in order to send a large amount of optical signals such as video signals at a high speed to the subscriber or to permit the subscriber to transmit and receive such information to and from another subscriber is one of subjects to be realized for an optical communication network. To this end, it is necessary to carry out processing of signals such as dropping or insertion of signals or cross-connection.
In optical subscriber networks proposed so far, a transmission processing node is provided on an optical highway in order to allow access between the optical highway and an optical subscriber. Such transmission processing node has such three basic functions as given below.
(1) A dropping function to extract desired data from the optical highway to a terminal of the optical subscriber.
(2) An inserting function to place data from the optical subscriber into an empty time slot of the optical highway to transmit the data.
(3) A cross-connecting function to exchange time slots or channels of data on the optical highway.
In an exemplary one of common transmission processing nodes proposed so far, such processes as described above are all based on processing of electric signals. More particularly, for example, in a dropping process, an optical signal on an optical highway is first converted into an electric signal by a photoelectric converting section at a transmission processing node, and then converted into an optical signal for a subscriber by an electro-optic converting section for the subscriber in order to transmit the signal to the subscriber. On the other hand, in an inserting process, an optical signal from the subscriber is converted into an electric signal by a photoelectric converting section and then joined with data of the optical highway, whereafter it is converted into an optical signal by an electro-optic converting section for the optical highway and sent into the optical highway.
However, in order to deal with an optical signal of a very high speed on an optical highway and besides achieve dropping, insertion and cross-connection or the like for multichannel optical subscriber lines based on electric processing, very high speed multichannel photoelectric and electro-optic processing sections and electric signal processing section are required. Consequently, signals to be treated are increased into a large amount, and accordingly, there are drawbacks that those processing sections may make a bottle neck in the phase of processing speed and that such processing devices are increased in scale as such channels increase.
An attempt to solve such drawbacks is disclosed in the following publication:
L. C. Blank et al: Demonstration of optical drop-and-insert for accessing 2.24 Gbit/s optical transmission systems directly at the 140 Mbit/s level, Technical digest of ECOC (European Conference on Optical Communication) '88, pp. 463-466.
A method of dropping and insertion disclosed in the publication is that a 2.times.2 optical switch is interposed intermediately of an optical highway such that it is switched at a predetermined timing to drop an optical data pulse of a desired time slot and insert another optical data pulse into the time slot. With the method, however, since the thus dropped data pulse remains at a high speed (2.24 Gbit/s in the example given), a receiving device including a high speed electronic circuit is essentially required in order to receive such optical data. Therefore, the method does not successfully solve the above described technical subject completely.